1. Field of the Invention
The invention relates to an optical fiber stress sensor and notably to a device enabling the detection of the stresses applied to an optical fiber. These stresses may be elongation stresses or compression stresses along the axis of the fiber, forces of pressure (shocks) applied perpendicularly to the axis of the fiber, strains (bending) and temperature variations that cause variations of the refraction index of the fiber.
This invention can be applied to the inspection of damage to structures, and especially to composite structures, by non-destructive techniques. It can be applied, for example, to the checking of constructive works (such as bridges and dams) and to the checking of strains undergone by aircraft frames.
The checking of damage to composite structures can be done by the insertion of optical fiber sensors. This method is described in the French patent application No. 91 15347. This method is especially well suited to the tracking of strains or to the detection of delamination under fatigue.
Damage to structures can be checked by the attachment by bonding, to the structure to be checked, of an assembly constituted by a rigid element and an optical fiber as described in the French patent application No. 92 03006.
All these systems have, in common, a method of measurement using a Michelson type interferometer.
The invention makes it possible to do away with the need for this optical interferometry by transferring the detection to the microwave field.
2. Description of the Prior Art
Among the many methods of measurement designed for the characterization of networks of optical fibers, two main categories may be distinguished:
reflectometry;
interfero-polarimetry.
Reflectometrical methods comprise methods of meaurement related to optical or microwave reflectometry in the time domain or the frequency domain. The best known of these methods are:
Optical Time Domain Reflectometry (OTDR)
Optical Frequency Domain Reflectometry (OFDR) and its derivatives, especially frequency gradient (FM-CW) type techniques.
Reflectometry is based on the principle of radar. A light pulse is injected into the optical fiber. The reflected light (i.e. the light backscattered from a defect of the fiber or a connector or a fold) is measured by means of a detector. The localization of the defects is obtained by measuring the time difference between the instant T.sub.0 at which the signal is sent and the instant T.sub.1 of return of the signal after reflection from the defect.
The main advantage of the OTDR lies in its relative simplicity and in its operation on every type of fiber.
The main drawbacks of this method are:
the degree of sensitivity in the detection of defects;
spatial resolution: indeed, OTDR remains simple so long as the spatial resolution required is not too great, i.e. so long as the minimum distance between two defects is not too small. If this distance is too small, then it is necessary to have available laser sources that are pulsed at high speeds (of the order of one picosecond for a resolution value, along the fiber, of the order of one centimeter).
Optical frequency domain reflectometry (OFDR) uses a technique very close to that of methods used for the analysis of networks in microwave mode.
In this technique, the optical carrier is frequency modulated. Then, the modulation of the carrier is made to vary by frequency hopping in a range of frequencies that is as wide as possible. The optical response of the system being tested is measured by heterodyne detection giving the amplitude and the phase for each frequency. The response in the time domain is obtained after Reverse Fourier Transform.
When the system works in microwave mode, with a wide passband (of several GHz), the spatial resolution obtained is great (values of the order of one mm. have been reported in publications).
The degree of spatial resolution is the main advantage of this technique.
The field of the frequency ramp or gradient technique comprises the applications of radar techniques to OFDR. In these techniques, the laser source is frequency modulated. A frequency ramp or gradient applied to the laser source enables the localizing of the defects of an optical fiber by measuring the delay between the transmission and the echo reeived by backscattering.
These techniques benefit from high spatial resolution, but require complex methods for the frequency modulation of the laser source (especially in the case of semiconductor lasers).
The interfero-polarimetrical methods are based on Michelson type optical interferometry.
This method is described especially in the patent application No. 88 00780.
The main advantage of this method comes from the sensitivity of the fiber, which is a fiber with hollowed structure, as a pressure or strain sensor, and from the use of interferometry.
The main drawbacks are:
the spatial resolution is presently limited to about ten centimeters;
the difficulty of implementing this method in a harsh environment.